This invention relates to a new and improved composition and process for removing scale or reducing scale formation in sewage plants. The composition and process are particularly suitable for use in anaerobic areas of a sewage plant such as in transfer lines from the system clarifier to areas such as sludge digestion and handling, including pumps, piping, valves, overflow lines, and possibly in centrifuges, etc.
These areas tend to be susceptible to precipitation of ions such as Ca, Fe, and Si, and the resulting formation of scale and struvite, especially under conditions of pH 5-9, and at temperatures from about ambient to exceeding 100 F.
In sewage treatment plants, struvite tends to occur in a bacteria laden and high solids environment, under conditions of agitation, and a pH of about 5-9. Scale, which occurs as a type of CaCO3, forms more frequently at a pH of about 7.2-8.4, especially where the Ca+2 content exceeds about 600 ppm. Given these unusual operating conditions and wide pH ranges, it is difficult to devise a suitable treatment for removing both scale and struvite.
Scale formation in sewage treatment plants has also been associated, in part, with vivianite and/or struvite, the latter being a magnesium ammonium phosphate hexahydrate, but this knowledge has not resulted in formulating a real solution to the deposition of CaCO3, vivianite or struvite, or for reducing scale generally in sewage treatment plants. Struvite deposition is discussed in THE BULLETIN, Winter 1991, printed by the California Water Pollution Control Association, and entitled: "Advances in Understanding Struvite Formation and Possible Methods of Control", by Robert McNabb.
Scale formation in boilers, and corrosion in water treatment plants such as cooling towers, boilers, heat exchangers, etc., has been prevented by using di-phosphonic acids; also, di-phosphonic acids are well known as cleaning and bleaching agents, corrosion inhibitors, etc. Moreover, acrylic acid terpolymers both alone, and in conjunction with di-phosphonates, have been used to prevent scale formation and to stabilize aqueous systems in these same areas. U.S. Pat. No. 4,711,725 discloses the use of acrylic acid terpolymers and interpolymers for this purpose. However, neither phosphonic acids either alone or together with terpolymers, have been known to remove scale generally, or struvite in particular, in sewage treatment plants.
U.S. Pat. No. 5,078,891 discloses the use of phosphonates with acrylic acid ether copolymers, and U.S. Pat. No. 4,640,793 discloses the use of a mixture or copolymer of a carboxylic acid with a sulphonic acid and a polycarboxylate phosphonate. But neither patent teaches the use of anionic groups in the polymers which are considered necessary to produce the desired descaling effects in sewage treatment plants.
The combination of phosphonohydroxyacetic acid and an acrylate copolymer functions to merely disperse (rather than actually dissolve CaCO3) in cooling tower descaling experiments. These latter tests were described in, "NEW ACRYLATE POLYMER FOR WATER TREATMENT PROGRAM", by Michael J. Ellis, William M. Mann, and, Judy M. Bardsley, published in RESIN, 1987, Volume XXXVII, No. 3, pages, 3-19, on page 9, Table V. Also as further shown in Table V, phosphonic acids such as phosphonohydroxyacetic acid alone will merely suspend, rather than dissolve CaCO3.
In addition, acrylic acid terpolymers do not appear capable of dissolving iron oxides and calcium phosphates and silicates in the form of hydroxyapatites and serpentines, but merely function to suspend these materials, as shown in Table VIII, page 16, of the RESIN publication, supra.